The present invention relates to chemical agent detection by ionization cells of the type generally described in U.S. Pat. No. 3,835,328. Such cells can be used to detect the presence of very minute concentrations of a select group of chemical vapors or gases in the air or in other vapor or gas backgrounds. Described in U.S. Pat. No. 3,835,328 is an ionization-type detector in which a gas sample flows past a source of ionization and through an attrition region to an ion collection region. A collector current is produced as a function of the concentration of positive ions and the concentration of negative ions in the gas sample reaching the collector region; the electrical potential of the collector and the rate of gas flow. By "attrition" in this invention is meant recombination in the flowing gas as well as removal from the gas to the walls by diffusions and turbulent transport.
By providing a long path and an exposure to adequate surface, the attrition of ions is enhanced and controlled in such a way that ion concentration, and in particular the difference between the concentration of positive ions and negative ions remaining in the gas sample when it reaches the collection region, is a function of trace gases in the sample. An improved gas ionization cell with compensation for variations in the flow rate of the gas and variations in radioactive source intensity is shown in U.S. Pat. No. 4,075,550 issued to Castleman et al and assigned to the same assignee as the present invention. This is accomplished by taking a portion of the gas sample, after it has been exposed to the ionizing radiation, and directing this portion of the gas sample past a pair of flow probes through a channel which bypasses the attrition region of the cell. The signal generated at the flow probes is proportional to the gas flow rate and the ionizing source intensity and is combined with the signal at the collector screen which is a function of the trace gas. A limitation is that although the prior art is capable of responding to the required low concentration, its use at high sensitivity settings is restricted because it also has a response to interferants. "Interferants" denotes any of a class of substances which causes an undesired signal, that is, a chemical noise. The response of such sensor cells as described above depends on two effects: the vapors in the air must form ions and the ions must survive transport through the cell.
In the present invention a "chemical bias," that is, a background concentration of a simulant, is provided to the inlet of an ionization detector of the general type shown in the references above and that is effective, by charge exchange, to prevent large concentrations of materials (interferants) such as jet fuel, gasoline, smoke, turpentine and the like from dominating the signal. The introduction of this chemical bias at the inlet suppresses the response of high concentrations of interference materials and thereby increases the useful "signal-to-noise" ratio of the chemical agent detectors.